The present invention relates to a semiconductor device and an electronic system having a plurality thereof disposed on a mounting board.
CPU boards have been developed having a microprocessor (CPU) and it coprocessors mounted thereon to form an electronic system. An example of such coprocessors is a floating point unit.
The semiconductor chips that include the microprocessor and coprocessors mentioned above are usually accommodated in a pin grid array (PGA) each. A semiconductor device of a PGA construction may have between tens and hundreds of external pins disposed around its periphery. In other words, the semiconductor device of the PGA construction is suitable for accommodating a semiconductor chip such as a microprocessor or a coprocessors dealing with numerous signals.
Electronic systems of this king incorporating microprocessors and coprocessors are illustratively found in pp. 123-138 of "Nikkei Electronics," of the July 13, 1987 issue.
The microprocessor and coprocessors to be mounted on the board of an electronic systems such as those cited above are each designed and developed separately. That is, the arrangement of external pins of semiconductor devices based on the PGA construction has not been standardized. For this reason, it has been impossible for the board of such microprocessors or coprocessors to dispose linearly, for example, the conductors carrying clock or coprocessors signals between the corresponding external pins. These conductors must be installed over long distances and/or disposed so as to cross other signal conductors. This has led to increases in both capacity and resistance of the signal conductors, causing delays in signal transmission. Because a CPU and its coprocessors perform synchronized transfer between them, clock skews that may occur between the CPU and any of its coprocessors or between coprocessors can cause a malfunction of the system and hamper high-speed operations thereof. In particular, because the clock and coprocessors signals are respectively the reference signal and the signal of the highest velocity in an electronic system, any delay in these signals necessarily reduces the overall performance of the entire system.
Delays in the above-mentioned coprocessor signals or clock signals disrupt the timing of the operation by the electronic system. This can result in system malfunction of the electronic system and reduce the electrical reliability thereof.
There has been no special consideration given to the arrangement of external pins of those semiconductor devices of the PGA construction which accommodate the above-described microprocessor and coprocessors. That is, the external pins that transmit the data bus and address bus signals over bus lines are arranged more or less randomly around the external pins for clock signals. When any of the data bus and address bus signals changes in signal level, the clock signals suffer from a noise by electromagnetic induction on the external pins for those signals. The noise distorts the waveform of the clock signals. Where he clock signals are of a low-frequency type, low levels of distortion of the clock signal waveform do not affect the performance in any appreciable manner. However, where a high-frequency signal is used in a system having a clock-synchronized bus, even very low levels of distortion in the clock signal waveform can trigger system malfunction. For example, in an electronic system that this inventor was developing, clock signals of as high as 40 MHz were adopted to implement higher operation speeds and more frequent bus cycle reiteration. Clock signals operating on such a high frequency to activate the microprocessor and its coprocessors involved led to frequent system malfunction.